On notification, the Nigeria Centre for Disease Control deployed a Rapid Response Team (RRT) to support outbreak investigation and response activities in the State. This study describes the epidemiology of and response to a large yellow fever (YF) outbreak in Edo State.
Nwachukwu William et al BMC Public Health https://doi.org/10.1186/s12889-022-14043-6 (2022) 22:1644 Open Access RESEARCH Epidemiological description of and response to a large yellow fever outbreak in Edo state Nigeria, September 2018 ‑ January 2019 E. Nwachukwu William1*, John Oladejo1, Chinenye Mary Ofoegbunam1, Chimezie Anueyiagu1, Festus Dogunro2, Sandra Okwudili Etiki2, Botson Iliya Dachung1, Celestina Obiekea1, Bukola Aderoju1, Kayode Akanbi3, Idayat Temitope Adeyemi3, Gboyega Adekunle Famokun3, Obi Emelife4, Irowa Williams Osamwonyi5, Chinwe Lucia Ochu1, Alice Abiode5, Faith Ireye6, Martins Chukwuji6, Oladipupo Ipadeola1, Musa Saiki7, Ifeanyi Okudo6, Dorathy Nwodo4, Joseph Avuwa Oteri4, Elsie Ilori1, Nwando Mba1 and Chikwe Ihekweazu1 Abstract Background: Edo State Surveillance Unit observed the emergence of a disease with “no clear-cut-diagnosis”, which affected peri-urban Local Government Areas (LGAs) from September to November 1, 2018 On notification, the Nigeria Centre for Disease Control deployed a Rapid Response Team (RRT) to support outbreak investigation and response activities in the State This study describes the epidemiology of and response to a large yellow fever (YF) outbreak in Edo State Methods: A cross-sectional descriptive outbreak investigation of YF outbreak in Edo State A suspected case of YF was defined as “Any person residing in Edo State with acute onset of fever and jaundice appearing within 14 days of onset of the first symptoms from September 2018 to January 2019” Our response involved active case search in health facilities and communities, retrospective review of patients’ records, rapid risk assessment, entomological survey, rapid YF vaccination coverage assessment, blood sample collection, case management and risk communication Descriptive data analysis using percentages, proportions, frequencies were made Results: A total of 209 suspected cases were line-listed Sixty-seven (67) confirmed in 12 LGAs with 15 deaths [Case fatality rate (CFR 22.4%)] Among confirmed cases, median age was 24.8, (range 64 (1-64) years; Fifty-one (76.1%) were males; and only 13 (19.4%) had a history of YF vaccination Vaccination coverage survey involving 241 children revealed low YF vaccine uptake, with 44.6% providing routine immunisation cards for sighting Risk of YF transmission was 71.4% Presence of Aedes with high-larval indices (House Index ≥5% and/or Breteau Index ≥20) were established in all the seven locations visited YF reactive mass vaccination campaign was implemented Conclusion: Edo State is one of the states in Nigeria with the highest burden of yellow fever More males were affected among the confirmed Major symptoms include fever, jaundice, weakness, and bleeding Majority of *Correspondence: drj_nwachukwuwe@yahoo.com; nwachukwu william@ncdc.gov.ng Nigeria Centre for Disease Control, Plot 800 Ebitu Ukiwe Street Jabi, Abuja, Nigeria Full list of author information is available at the end of the article © The Author(s) 2022 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ The Creative Commons Public Domain Dedication waiver (http://creativeco mmons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data Nwachukwu William et al BMC Public Health (2022) 22:1644 Page of 13 surveillance performance indicators were above target There is a high risk of transmission of the disease in the state Low yellow fever vaccination coverage, and presence of yellow fever vectors (Ae.aegypti, Ae.albopictus and Ae simpsoni) are responsible for cases in affected communities Enhanced surveillance, improved laboratory sample management, reactive vaccination campaign, improved yellow fever case management and increased risk communication/awareness are very important mitigation strategies to be sustained in Edo state to prevent further spread and mortality from yellow fever Keywords: Yellow fever, Outbreak, VPD, Edo-state Introduction Yellow fever (YF) is an acute viral haemorrhagic disease characterised by fever, yellowness of the eyes, skin and urine caused by the yellow fever virus which belongs to the genus Flavivirus It is a vector-borne (arbovirus) disease transmitted to man from the bites of infected Aedes mosquitoes Humans and non-human primates are the reservoir hosts of yellow fever [1, 2] There are three basic transmission cycles of yellow fever: i) the jungle (sylvatic) yellow fever virus transmission cycle, is between nonhuman primates (e.g monkeys) and mosquito species; ii) The intermediate (savannah) cycle involves the transmission of the virus from infected mosquitoes to humans living or working in jungle border areas; iii) the urban cycle involves transmission of the virus between humans Transmission occurs when an infected person from the jungle or savannah introduces the disease in human populations with low immunity for fellow fever [3, 4] Yellow fever remains a public health problem, especially in Africa, despite the availability of an effective vaccine [5] This is due to several factors including uncontrolled urbanisation with rapid encroachment into natural habitats of the yellow fever vector, low production capacity for yellow fever vaccines and limited enforcement of the International Health Regulations (IHR) by countries in the region [1, 2] Yellow fever control is guided by the World Health Organization (WHO) coordinated “Elimination of Yellow Fever Epidemic (EYE) Strategy” with three strategic objectives namely: to protect at-risk populations (no epidemics), prevent international spread (no exportation) and contain outbreaks rapidly (no sustained transmission) [6] The current cycle of yellow fever transmission in Nigeria was detected in September 2017, in Ifelodun Local Government Area (LGA) of Kwara State, 21 years after the last reported confirmed yellow fever case [7] Since the onset of the outbreak, increasing numbers of cases with increasing geographic spread have been reported From July 2017 to December 2018, a total of 163 confirmed cases in 46 LGAs in 17 states were reported from the Institut Pasteur (IP Dakar), Ninety deaths were reported (CFR = 2.2%) from all suspected cases and 31 deaths among confirmed cases (CFR = 19.0%) [8, 9] As a major preventive measure, the yellow fever vaccine was introduced into routine immunization (RI) schedule nationwide in 2004 targeting children 9 months to years However, since the re-emergence of YF in 2017, yellow fever vaccinations have been accelerated through both preventive and reactive mass vaccination campaigns As at first quarter of 2021, 11 international coordinating group for vaccine provision (ICG) requests were approved for YF reactive mass vaccination (RMVC) with about 15 million Nigerians vaccinated across 75 LGAs in 16 States While 88,121,329 were vaccinated through preventive mass vaccination campaigns in 19 states Planned phase PMVC schedule to cover all the states in Nigeria till 2025 [10] The first recent confirmed case of yellow fever in Edo State was recorded in May 2018 of a 65-year-old woman from Etsako East LGA The case presented with fever, jaundice and vomiting with no history of vaccination nor travel to yellow fever affected states On November 14, 2018, the Epidemiology Unit of the Department of Disease Control, Edo State reported an observed incidence of a disease with “no clear-cut diagnosis” that required urgent attention, to the Nigeria Centre for Disease Control (NCDC) The cases presented with clinical signs and symptoms suggestive of a viral haemorrhagic disease, with dates of onset between September and November 1, 2018 The cases were resident in four LGAs including Esan Central, Esan West, Owan East and Uhunmwode The state has the highest Lassa fever (LF) burden in Nigeria [11] Most LGAs with high LF burden were the LGAs that reported the strange disease This led to a low index of suspicion of yellow fever in the affected communities which consequently increased the mortality experienced in the outbreak Preliminary investigations on the samples for Lassa fever at the Institute of Lassa Fever Research and Control (ILFRC), Irrua, Edo State were negative Further investigations were done using IgM serology in Central Public Health Laboratory Lagos (CPHL) and metagenomic analysis at the African Center of Excellence for Genomics of Infectious Diseases (ACEGID), Redeemer’s University, Ede, Osun State The results of these investigations were positive for yellow fever [12] Following the notification Nwachukwu William et al BMC Public Health (2022) 22:1644 of these cases, NCDC deployed a multi-disciplinary team to support the state’s response to the outbreak The objectives of the deployment were to describe the re-emergence of yellow fever, assess the risk of a larger outbreak occurring, assess the determinants of the outbreak and define short, medium- and long-term control measures The aim of this study is to provide the descriptive epidemiology of and response to a large yellow fever (YF) outbreak in Edo State Methods Study area/ study design This is a cross-sectional descriptive outbreak investigation and response of yellow fever in Edo State, Nigeria as at January 2019 Edo State is one of the states in the South-South geo-political zone of the country with 18 LGAs [13] The clusters of “cases of a strange illness” that initially affected four LGAs namely, Esan Central, Esan West, Owan East and Uhunmwode LGAs, later increased in both severity and geographic coverage extending to 12 LGAs between September 2018 and January 2019 Advocacy visits were paid to key stakeholders to provide information about the presence and purpose of the team in the state and to obtain detailed information on the current situation and activities undergone At the community level, advocacy visits were made to community leaders by the RRT detailing the nature and risks associated with the disease and preventive measures Community leaders were sensitised on the case definition for yellow fever Operational case definition A modified standard case definition for YF from the integrated disease surveillance and response (IDSR) technical guidelines (2013) for Nigeria was adapted as the working case definition and utilised for the purpose of identifying suspected cases of YF residing in the communities in Edo State [14] The study population included persons who met the case definitions of yellow fever as follows: i Suspected Case: Any person residing in Edo State with acute onset of fever, with jaundice appearing within 14 days of onset of the first symptoms with or without bleeding from September 1, 2018 to January 12, 2019 ii Probable Case: A suspected case whose sample was IgM positive / PCR positive/metagenomics positive in a national laboratory in the absence of YF vaccination within 30 days of onset of illness with an epidemiological link to a confirmed case or an out- Page of 13 break and positive post-mortem liver histopathology iii Confirmed Case: A probable case and the detection of YF-specific IgM, detection of a four-fold increase in YF IgM and/or IgG antibody titres between acute and convalescent serum samples, detection of YFV-specific neutralising antibodies at WHO Regional Reference Laboratory, Institut Pasteur Following the establishment of case definitions for the outbreak, the activities detailed below were subsequently carried out during the outbreak investigation: i Active case search Active case search was done in line with the YF preparedness and response guideline and YF field investigation guide [15, 16] Active case search was conducted by the RRT at the health facilities and communities For health facilities (HF), a retrospective review of HF records (registers/case notes) took place at the medical records, outpatient and inpatient and the laboratory sections from September 1, 2018 to January 12, 2019 was done Patients who met the case definitions were added to a specific yellow fever outbreak line list Two approaches were used in the community active case search Community leaders were sensitised on the case definition for yellow fever The first approach was to assemble community members together in a place approved by the community leader where they were sensitised and examined for symptoms and signs of YF The second approach was a house-to-house case search where the RRT visited every house in the community with an assigned community guard by the community leader Any person that met the case definitions for suspected case was added to a line-list and their blood sample collected Detailed case investigation was carried out on all the confirmed cases Human blood sample management The RRT facilitated sample management (collection, packaging, and transportation) as part of outbreak response activities All suspected cases had 5mls of venous blood collected by the laboratory team The samples stored in plain bottles were centrifuged at 500 g-1000 g for to obtain sera The sera were collected into cryovial tube(s), stored at + 2 to + 8 °C or frozen at − 20 °C degrees Celsius (°C) These samples were triple packaged and shipped under good cold chain through a contracted courier company to the NCDC Central Public Health Laboratory (CPHL), Yaba, Lagos for IgM serology Positive (presumptive positive) samples were sent to the World Health Organization (WHO) Regional Reference Laboratory, Institut Nwachukwu William et al BMC Public Health (2022) 22:1644 Pasteur (IP) Dakar where both real-time polymerase chain reaction (RT-PCR) and plaque reduction neutralization test (PRNT) were used for final confirmation ii Risk assessment Risk assessment was done at the state level using a set of 14 criteria for the assessment: Each criterium was given a maximum score of one and a minimum score zero (1 or 0): Total score was 14 while least score was Earned score was divided by the total score and multiplied by 100 The percentage scores were graded thus: 70-100% is very high risk; 40-69% is moderate risk and below 40% is low risk Data were collected using a pro forma, entered and analysed using Microsoft Excel In addition, a risk communication gap assessment to review existing documents and reports, inventory of existing communication materials and key informant interviews Coordination and system strengthening, yellow fever jingle, media plan, training schedule for healthcare workers and community engagement were carried out iii Verbal autopsy Verbal Autopsy (VA) was used to estimate disease burden, mortality, and under-reporting of yellow fever as part of the National Yellow fever Outbreak Response Strategy A case of VA was defined as “any death of a family member(s) who prior to death developed acute onset of fever and jaundice appearing within 14 days in a person who resided in Uhunmwode, Esan West, Esan Central and Owan West or any other LGAs within Edo State between September 1, 2019, to January 12, 2019” [7] A questionnaire was used to collect data from family members Any death in the community that met the case definition was included However, all cases line listed in the VA were verified with the state surveillance data Those already captured in the state surveillance data were excluded from the report iv Entomological surveillance An entomological survey was conducted in the first four LGAs to identify the presence of the yellow fever vectors The approaches used to establish the presence of the vectors, Aedes mosquitoes, in the locations visited include (i) larval sampling, which was designed to collect immature stages (larvae and pupae) of the vectors (ii) Ovitraps were designed to collect Aedes mosquito eggs (iii) modified Human Landing Catch (mHLC), designed to collect adult mosquitoes Two types of adult collection traps were deployed: Biogents’-sentinel trap and CDC UV light trap [17–19] Page of 13 xxii Rapid Vaccination Coverage Assessment Rapid Vaccination Coverage Assessment (RVC) was conducted in the four LGAs where the outbreak started to determine the yellow fever vaccination status of children 10 years and below in the community, as part of the national YF outbreak response strategy A systematic sampling of alternate houses was used to identify those to be included The assessment began where the RRT met with the community leader and the team subsequently moved in a clockwise direction Children below the age of one and above 10 years were excluded A living first-born child between and 10 years in each house was studied until 10 children per settlement were identified and their caregivers interviewed A caregiver at each selected house was asked for the history of yellow fever vaccination as well as documentary evidence in the routine immunisation (RI) cards to show that the child had YF vaccination Sighting of the immunisation card and date of yellow fever vaccination was evident that the child received YF vaccination vi Yellow fever reactive mass vaccination campaign A request for YF reactive mass vaccination campaign was made through the International Coordinating Group (ICG) for vaccine provision Upon approval by the ICG, pre-implementation and implementation microplans were developed The campaign strategy was a fixed and temporary fixed post campaign strategy targeting the age groups of 9 months to 44 years (85% of total population) vii Data management and Analysis Yellow fever specific investigation data tools were used for different activities, and these include Active case search: the yellow fever specific line-list in Excel template was used and analysed with Microsoft Excel software Verbal autopsy: data was collected using a structured-interviewer-administered questionnaire Data was entered and analysed using Epi-Info software Risk assessment: Checklist was used for data collection and analyses with Microsoft Excel software Entomology: A customized excel template was used in collection of entomology data Rapid yellow fever vaccination coverage assessment: a checklist was used to collect data Data was entered and analysed using Epi-Info software RMVC data were collated and analysed using the yellow fever mass vaccination campaign database in Microsoft Excel All data analysis done were descriptive data analysis using percentages, proportions, and frequencies Nwachukwu William et al BMC Public Health (2022) 22:1644 Table 1 Summary of demographic characteristics of suspected yellow fever cases in Edo State, September 2018–January 2019″ Demography and clinical characteristics of Frequency (N = 209) Percentage (%) Sex Male 159 (76.1) Female 50 (23.9) Age (years) Age range 70 years (1 – 71 years) Median age 20 years Affected LGAs Uhunmwode 72 (34.4) Result Presumptive positive 94 (45.0) Confirmed 67 (32.1) Male 51 (76.1) Female 16 (23.9) Not a casea 142 (67.9) Deaths Suspected 25 (12.0) Presumptive 17 (18.1) Confirmed (IP Dakar) 15 a Not a case: All negative cases at both National and Regional Reference Laboratories excluding IP Dakar confirmed cases Page of 13 Results Demographic characteristics of study participants Two hundred and nine (209) suspected cases of YF were recorded from 16 LGAs during the active case search in the communities and the retrospective record review of data from health care facilities from September 1, 2018– January 12, 2019 Tables and The outbreak started from peri-urban LGAs namely: Uhunmwode, Esancentral, Esan-West and Ovia North-East and these LGAs had both the highest number of cases and attack rate per 100,000 population Seventy-two (34.4%) of the cases were from Uhunmwode LGA (Figs and 2) The ages of the suspected cases were between to 71 years [median: 20 years and range (70 years)], 159 (76.1%) were males and 50 (23.9%) females in a ratio of 3:1 (Table 1) The yellow fever outbreak affected more males within the age group of 11-30 years (Fig. 3) About 94 (45.0%) of the suspected cases were presumptive positive/inconclusive (IgM+) cases and 67 (32.1%) of Institut Pasteur Dakar confirmed cases were recorded See Fig. 4 showing map of LGA distribution of cases Blood samples of 174 (83%) cases were collected and sent to the laboratory Twenty-five (12.0%) deaths were recorded from suspected cases, 17 (18.1%) deaths were recorded from presumptive positive cases (Table 1) Among confirmed cases, the 67 (32.1%) confirmed cases, were reported from 12 LGAs Male to female ratio is 3.2:1 Fifty-one (76.1%) were males; median age was 24.8, range 63 (1-64) years and 13 (19.4%) had history of Table 2 Classification of cases of yellow fever by LGA in Edo State from September 2018–January 2019 Yellow Fever Cases in Edo State September 2018 - January 2019 Affected LGA Number of Suspected Cases of YF (%) Number of Presumptive Cases of YF (%) Number of Confirmed Cases of YF (%) Akoko Edo (0.5) (1.1) (1.5) Egor (3.3) 0 Esan-Central 15 (7.2) (6.4) (4.5) Esan North-East (4.3) (5.3) (6.0) Esan-West 37 (17.7) 19 (20.2) 14 (20.8) Etsako East (0.5) 0 Etsako West 15 7.2) (7.4) (7.5) Iguegben (3.3) (2.2) (3.0) Ikpoba-Okha 11 (5.3) (7.4) (6.0) Oredo (1.4) (2.2) (3.0) Orhionmwon (0.5) 0 Ovia North-East 17 (8.2) (7.4) (10.4) Ovia South-West (0.5) 0 Owan-East (3.8) (3.2) (3.0) Owan-West (1.9) (3.2) (4.5) Uhunmwode 72 (34.4) 32 (34.0) 20 (29.8) Total 209 (100.0) 94 (100.0) 67 (100.0) Nwachukwu William et al BMC Public Health (2022) 22:1644 Page of 13 Fig. 1 Suspected yellow fever cases in Edo State by LGAs September 2018 – January 2019 Fig. 2 Yellow fever attack rate by LGA in Edo State September 2018 – January 2019 YF vaccination Fifteen deaths [Case fatality rate (CFR 22.4%)] were recorded Figure shows the epicurve of the yellow fever outbreak and the timeline of response activities carried out during the outbreak The height of the epicurve increased following enhanced active cases search which led to increase in case detection and the sharp drop in the epicurve following commencement of yellow fever vaccination in the affected LGAs Yellow fever surveillance performance indicators were measured and compared with WHO standard See Table 4 Table four describes set of evaluating standards used to ensure that YF surveillance can meet the objectives of its surveillance system About (85.7%) of the performance indicators were achieved within the reporting period However, (14.3%), was lower than the target due to incomplete documentation of the date of release of laboratory result Nwachukwu William et al BMC Public Health (2022) 22:1644 Fig. 3 Age-sex distribution of yellow fever cases in Edo State September 2018 - January 2019 Fig. 4 Map of Edo State showing distribution of suspected and confirmed yellow fever cases by LGA September 2018 – January 2019 Page of 13 Nwachukwu William et al BMC Public Health (2022) 22:1644 Page of 13 Fig. 5 Epi curve of yellow fever cases in Edo State September 2018 - January 2019 Rapid vaccine coverage assessment Summary of risk assessment The yellow fever rapid vaccination coverage assessment was carried out in the first four LGAs that reported yellow fever cases Two hundred and forty-one (241) children were assessed, targeting children aged - 10 years [in Uhunmwode -178 (73.9%), Esan West - 47 (19.5%), Esan Central - 13 (5.4%), Ovia North-East - (1.2%)] Males accounted for 51.9% and females, 48.1% Immunisation cards were available in 44.6% respondents and were completely filled in 33.7% of respondents History of yellow fever vaccination was reported among 94 (39.0%) Yellow fever vaccination status among children that produced immunisation cards in all four affected LGAs was 81 (33.7%) When compared with other RI antigens, yellow fever vaccination had the lowest number of children vaccinated Table presents the outcome of the risk assessment of yellow fever transmission conducted in Edo State during the outbreak response Overall, the risk of yellow fever spread in the state was high (71.4%) based on the assessment criteria Verbal autopsy There were cases of deaths recorded from the communities in affected LGAs associated with yellow fever based on the case definition Four deaths were recorded from two LGAs that were not captured by the state surveillance team; Uhunmwode (2), Ovia North-East (2) in which two were male and two females All four cases presented with fever and jaundice before death Entomology survey A total of 355 larval containers were inspected from 170 houses in different settlements in the four LGAs in Edo state Of these, 36.5% of the houses were positive for Aedes larvae while 28.5% of water retaining containers were positive for various immature stages of Aedes species The immature stages of Aedes mosquitoes were collected mainly in abandoned domestic water containers and in leaf axils of banana plantation in all the LGAs sampled Presence of vectors of yellow fever (Ae.aegypti, Ae.albopictus and Ae.simpsoni) were established in all LGAs sampled High larval index was observed as shown by House and Breteau index See Table 6 Reactive mass vaccination campaign Following the ICG request and approval on the December 12, 2018, reactive vaccination campaign was implemented in 13 (72.2%) of the 18 LGAs on December 18, Nwachukwu William et al BMC Public Health (2022) 22:1644 2019 These LGAs had confirmed yellow fever case(s) or contiguous with a LGA that had confirmed case A total of 1,734,423 persons were vaccinated during the campaign in the 13 LGAs that implemented the yellow fever RMVC Ten (76.9%) LGAs had administrative coverage above 95% benchmark needed to achieve herd immunity for yellow fever infection See Fig. 6 Discussion The Edo State YF outbreak is the most severe outbreak since the re-emergence of YF in Nigeria in 2017, defined in terms of number of cases, geographic spread, and mortality The burden of the disease was higher than initially reported and the risk of transmission in the state was high as shown in the rapid risk assessment In 2017, Kwara State had a similar occurrence of a YF outbreak [7, 20] The transmission could be described to be of sylvatic origin because of the presence of forests and non-human primates (NHPs) which later spread to semi-urban and urban areas We have observed that majority of yellow fever outbreaks in Nigeria could be of sylvatic transmission as observed in Kwara, Kogi and Zamfara and other affected states However, the Edo State yellow fever outbreak transited from sylvatic to urban transmission Page of 13 An important consideration which influences the transition cycle from sylvatic to urban is internal migration The most affected areas in Edo State are rural and periurban areas with distances between 40 km to 90 km to urban areas of Benin City, with substantial daily traffic WHO has observed that with increased virus circulation and intense population migration from infected forest areas to urban settings, many large cities were burdened by yellow fever epidemics in West African countries and simultaneously, many smaller cities are still exposed to the disease [21] This serves as a critical indicator to intensify efforts in controlling outbreaks in rural areas, to avoid wider urban yellow fever transmission which has more devastating effects There was a delay in reporting the outbreak due to poor index of suspicion of yellow fever among clinicians Edo State is one of the states with the highest burden of Lassa fever in Nigeria [11] This may have contributed to the delay in diagnosis as most of the initial YF cases had similar presentations with Lassa fever which has higher index of suspicion among clinicians in the state Similar presentations were seen in Kwara [7] and Kogi yellow fever outbreaks Again, with high burden of both Lassa fever and yellow fever in the state, the concept of co-circulation of multiple viral haemorrhagic fever (VHF) diseases needs Fig. 6 Yellow fever administrative coverage during yellow fever reactive mass vaccination campaign in Edo State December 2018-January 2019 Nwachukwu William et al BMC Public Health (2022) 22:1644 Table 3 Presenting symptoms of confirmed cases of yellow fever in Edo State September 2018 – January 2019 Presenting symptoms among confirmed cases Number of confirmed cases [(n = 67), Percent (%) Fever 40 (59.7) Weakness 27 (40.3) Jaundice 24 (35.8) Bleeding 20 (29.9) Vomiting 19 (28.4) Head & Dizziness 15 (22.4) Abdominal pain 13 (19.4) Dehydration 12 (17.9) Unconscious (13.4) Restlessness (13.4) Irrational talking (13.4) Poor appetite (7.5) Page 10 of 13 further investigation [22] This necessitates the need for training of health care providers on the epidemiology and surveillance of yellow fever at all levels of health care system to improve early detection and underscores the need for the introduction of multi-pathogen kits for the diagnosis of VHFs [23] From our study, the male gender is more in number than the female and the predominant age group affected is 11 – 30 years The gender and age groups implicated are the most productive and adventurous set in the population They are more involved in out-door activities and occupations Control programmes should target this population group Table presented most of the symptoms observed among confirmed cases of yellow fever during this outbreak In line with Simon, Hashmi and Torp observation, in their documentation on complications of YF, identified Table 4 Summary of yellow fever performance indicators for surveillance in Edo State from September 2018–January 2019 Yellow fever performance indicators for surveillance Maximum Target (%) Performance Percentage of LGA reporting (Total number of LGAs in Edo =18 LGAs) Percentage of LGAs that collected blood samples from at least one suspected case of yellow fever per year: target ≥80% (n = 16) Percentage of all suspect cases for which specimens were collected: target ≥50% (N = 209) Percentage of cases investigated within 48 hours of notification: target ≥80% (n = 189) Percentage of samples sent to the laboratory within three days of investigation: target ≥80% (n = 189) Percentage of samples reaching laboratory in adequate/good condition: target ≥80% (n = 189) 18 > 80 16 (88.9) 18 > 80 16 (88.9 209 189 189 189 For IgM test: laboratory results reported < seven days after receipt of blood specimen: target ≥80% (n = 189) 189 ≥50 ≥80 ≥80 ≥80 ≥80 189 (90.4) 167 (88.4) 154 (81.5) 157 (83.1) 82 (43.4) Table 5 Rapid yellow fever risk assessment analysis for Edo State September 2018–January 2019 Description Risk Response Score Any suspected case(s)? Yes Yes Any presumptive positive case(s)? Yes Yes Any IP Dakar confirmed case(s)? Yes Yes Sharing international borders? Yes No Presence of International Airports in the state? Yes No Sharing border with state(s) that have reported outbreak yellow fever? Yes Yes Presence of any center/office for adult YF vaccination in the state? No Yes a Any known forest in the state/presence of NHP? Yes Yes City greater than 250,000 population in the state? Yes Yes Any security compromised area in the state? Yes No RRT Rapid YF vaccination coverage